Most semiconductor devices make use of several different levels of metallization. With the increasing complexity of devices and the need to reduce the physical size of devices, the number of metal interconnect levels is increasing. In addition, with the desire to increase the speed of the devices while reducing the power consumed by the devices, advanced metallization schemes are being developed. One such scheme involves the use of copper for the bus lines and interconnects. To improve the speed performance of the interconnect lines, interlevel dielectrics with lower dielectric constants than standard silicon dioxide films may be used as the insulating material situated between metallic structures. One such low dielectric constant interlevel dielectric material is OSG (organo-silicate glass).
Prior to the use of Cu for interconnection metal, aluminum metallization schemes used a standard, isotropic oxygen plasma etch to remove photoresist after a via or trench dielectric etch process. Unfortunately, it was observed that when Cu was used for the metallization, the etch removal of the photoresist with oxygen plasma at the 250° C. to 300° C. substrate temperatures typically used for Al metallization undesirably caused a substantial amount of oxidation to any exposed copper at the bottom of the via structures. It was also observed that etching the photoresist with substantial amounts of oxygen caused an undesirable reaction of oxygen with the Si-C bonding in the dielectric layer. When the dielectric layer is OSG, the material contains from 10-25% of C bound to Si. The removal of the Si-C bonding occurs when O2 gas is used in plasmas exposed to OSG, and occurs for isotropic (250° C.-350° C.) or anisotropic (from room T to 300° C.) plasma processing. Removal of the Si-C bonding leads to an increase in the dielectric constant from approximately 2.9 for some forms of OSG, to close to 4.0, which is the dielectric constant of SiO2. Accordingly, the use of oxygen to etch the photoresist dispenses with many of the benefits of using low dielectric constant interlevel dielectric materials.
In turn, the industry moved away from O2 based photoresist etches to hydrogen-based photoresist etches, especially in those situations where copper metallization schemes and low dielectric constant interlevel dielectrics were being used. The original hydrogen based photoresist etches were conducted at relatively high temperatures (e.g., temperatures ranging from about 225° C. to about 350° C.). In certain situations, however, the relatively high temperatures caused an undesirable large amount of residue formation on the copper metallization structure.
Accordingly, it was discovered that lowering the temperature (e.g., to a substrate temperature of around 150° C.) of the hydrogen based photoresist etches would reduce the amount of residue formation on the copper metallization structure to an allowable amount that could be handled by various clean up steps. While the lower temperatures created less residue formation on the copper metallization structure, it also substantially reduced the etch rate of the hydrogen based photoresist etches. Unfortunately, in certain circumstances the lower temperature reduced the etch rate of the hydrogen based photoresist etches to a value too low to be useful in a practical manufacturing situation.
Accordingly, what is needed in the art is a hydrogen-based photoresist etch that accommodates the desires of the industry without experiencing the drawbacks of the prior art processes.